Oxidation of ethanol

The experiment is most appropriate for post-16 students.This is a straightforward class experiment that will take about 10 minutes.

Equipment

Apparatus

• Goggles
• Boiling tube (note 1)
• Dropping pipette
• Test tube rack
• Test tube holder
• Bunsen burner
• Heat resistant mat

Chemicals

• Ethanol (IDA, Industrial Denatured Alcohol) (HIGHLY FLAMMABLE, HARMFUL), 2–3 cm³, provided in small bottles
• Acidified sodium dichromate(VI) solution, 10 cm³ (VERY TOXIC, CORROSIVE) (note 2)

Equipment notes

1. A large, 150 x 25 mm, test tube.
2. Student should be provided with the acidified dichromate(VI) solution already prepared as described here. Wearing goggles (or a face shield), prepare the solution by completely dissolving 25 g of sodium dichromate(VI)–2–water (Na₂Cr₂O₇.2H₂O) in 75 cm³ of water in a beaker or conical flask. Slowly and carefully add 10 cm³ of concentrated sulfuric acid and mix thoroughly. Provide this solution in small, appropriately labelled bottles with pipettes. Students should be instructed in the safe use of this solution.

Health, safety and technical notes

• Read our standard health and safety guidance
• Wear goggles. 
• Ethanol or IDA (Industrial Denature Alcohol), C₂H₅OH(l), (HIGHLY FLAMMABLE, HARMFUL) – see CLEAPSS Hazcard HC040a.
• Acidified sodium dichromate(VI) solution, Na₂Cr₂O₇(aq) (VERY TOXIC, OXIDISING, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC078c and note 2. 
• Concentrated sulfuric acid, H₂SO₄(l) (CORROSIVE) – see CLEAPSS Hazcard HC098a.  

Procedure

1. Place about 3 cm³ of acidified sodium dichromate solution in a boiling tube.
2. Use a teat pipette to add 5–7 drops of ethanol, with shaking.
3. Cool the mixture in the tube under a tap. Note the sweetish smell of ethanal (acetaldehyde) at first, then becoming sharper as oxidation continues and forms ethanoic acid (acetic acid).
4. When the reaction has subsided, the mixture can be warmed gently; the smell of ethanoic acid will become more noticeable.

Teaching notes

This experiment would usually form part of the teaching and learning sequence for the chemistry of alcohols, aldehydes and acids. The colour of the solution turns from orange to blue-green as the dichromate(VI) ion, Cr₂O₇^2-, is reduced to the chromium(III) ion, Cr³⁺.

Note that the sodium dichromate solution is more concentrated than 0.2 mol dm^-3, so the toxicity level is high, and students need to be instructed in safe handling. Consider wearing protective (nitrile) gloves. A more dilute solution can be used if preferred, with longer reaction time – the mixture may need gentle heating in a water bath.

Ball and stick molecular models will be useful for modelling both stages of the reaction scheme.

Students could be asked to write equations for the reactions involved when ethanol is oxidised by sodium dichromate in acid solution to (i) ethanal (ii) ethanoic acid, using [O] to represent oxygen coming from the dichromate.

A variant on this experiment as a demonstration for advanced students is to place four petri dishes on an overhead projector (protected from spillage with ‘cling’ film), and pour the same volume of acidified sodium dichromate solution into each dish. Then add 1 cm³ of 3 different alcohols, one to each dish (leave the fourth dish is a control), and observe the different rates of oxidation from the rate of the colour change. This approach can be used to investigate the effect of chain length of primary alcohols, or the differences between the reactions of oxidising agents with primary, secondary and tertiary alcohols.

The ‘breathalyser’ reaction is a nice linked experiment demonstrating a real world application of this type of chemistry. This reaction can be easily adapted to analyse alcoholic drinks for their ethanol content.